We've actually been looking into this for our application, since we want the browser to render the file embedded if it is capable of doing so - and if not capable, we want the browser to download the file under an appropriate name.
If you change the Content-Disposition header to be inline instead of attachment, it works in that manner - the browser will render the file if it is able to do so, and if not, the file will be downloaded as whatever you specify in the filename portion of the Content-Disposition header
response.headers['Content-Disposition'] = "inline; filename=name.extension"
However, if as you have said, your blah.csv comes back with that header and you can't intercept it or change that, then I would agree there is no way around it. The "attachment" part specifies that the file be downloaded.